1. Field of the Invention
The present invention relates to a semiconductor circuit, and particularly to a push-pull wideband amplifier used to amplify signals in, for example, CATV (Cable Television) systems.
2. Description of the Related Art
Such fields as CATV (cable television) require wideband amplifiers that amplify signals across a broad frequency bandwidth. Wideband amplifiers for CATV systems are generally configured as HICs (hybrid integrated circuits).
The push-pull type is one circuit configuration of a wideband amplifier. The hybrid push-pull wideband amplifier described in Japanese Patent Laid-open No. 52407/91 (JP, 03052407, A) is one example of an HIC wideband amplifier for a CATV system. FIG. 1 is a circuit diagram showing the configuration of the push-pull wideband amplifier described in JP 03052407, A.
This wideband amplifier is constructed with two amplifying circuit units 140 and 150 which operate in a mutual push-pull relation. The two amplifying circuit units 140 and 150 are both cascade amplifying circuits, are mutually connected, and have essentially the same internal circuit configurations. The first amplifying circuit unit 140 is made up of resistors R141 to R145, capacitors C141 and C142, and transistors Tr141 and Tr142; and the second amplifying circuit unit 150 is made up of resistors R151 to R155, capacitors C151 and C152, and transistors Tr151 and Tr152. Capacitor C164 is an RF (radio frequency) bias capacitor, and resistor R164 and capacitor C163 are optional. Resistor R161 and capacitor C162 provided between amplifying circuit units 140 and 150 receive the effect of an imaginary grounding, and serve as a means of independently controlling the AC (alternating-current) emitter impedance of transistors Tr141 and Tr151.
In a circuit configured according to the foregoing description, input signals are supplied to each of amplifying circuit units 140 and 150 in push-pull mode by RF input transformer T161, and the signals amplified by amplifying circuit units 140 and 150 are extracted by way of RF transformer T162.
The DC (direct-current) power supply (V.sub.cc) is supplied to each of amplifying circuit units 140 and 150 by way of resistors R162 and R163 and RF output transformer T162. Output shunt capacitor C165 is optional, but assists in controlling high-frequency gain and output impedance of the circuit.
Owing to their excellent high-frequency characteristics, bipolar transistors are typically used for transistors Tr141, Tr142, Tr151, and Tr152, which are the amplifying means, but other types of components for RF amplifications, ICs, or subassemblies may be used.
Finally, optional capacitors (not shown) between a ground point and the bases of transistors Tr141 and Tr151 are provided to assist matching of input impedance.
Although a CATV HIC wideband amplifier having a push-pull circuit generally gives rise to composite second-order (CSO) distortion, the above-described semiconductor circuit of the prior art does not include a circuit for compensating CSO, CSO deterioration being suppressed merely by the balance of the elements and electrical characteristics of each of the push-pull circuits.
Although such methods as adjusting the windings of the transformer or adjusting the balance between each port of the transformer can be considered as methods of compensating CSO in the circuit shown in FIG. 1, these methods are not amenable to automation and entail extra time and trouble, resulting in higher costs. In addition, the circuit constants of resistor R162 and capacitor C163 along with resistor R163 and capacitor C164 are determined by bias conditions alone and are not selected by high-frequency termination characteristics. The resistances of the resistors therefore vary widely according to the bias constants of the transformers. Resistance as high as, for example, 100 .OMEGA. or more results in high-frequency high impedance, while resistance as low as, for example, 10 .OMEGA. or lower results in a high-frequency short-circuited state due to capacitors C163 and C164. The contingencies for setting bias generally results in resistances of several hundred ohm or more in HIC for CATV, resulting in total reflection state in the high-frequency region.
If the push-pull circuit goes out of balance due to, for example, variation between elements, imaginary ground point (point A in the figure) only loses its grounded status, and a minute high-frequency oscillation in potential is generated at the imaginary ground point. This oscillation then becomes an oscillation source and a high-frequency current flows, but since the circuit coupled to the imaginary ground point acts as a bias circuit, the generated oscillation is totally reflected, and a progressive wave and reflected wave are generated. A standing wave is generated by this progressive wave and reflected wave, and this standing wave causes increased oscillation of the connected gate potential. This potential fluctuation is amplified, causing further deterioration of the balance of the push-pull circuit.